Along with the technical evolvement and emerging data services, such a scenario may occur in later releases of the Long Term Evolution-Advance (LTE-A) system that a user equipment is configured to operate while being served by a plurality of base stations (i.e., dual connectivity).
For example, a user equipment is configured with a Master Evolved NodeB (MeNB) and at least one Secondary base station (SeNB), where in the dual connectivity application scenario, at least an S1-MME (wherein MME stands for a Mobility Management Entity (MME), and S1 stands for an S1 interface to the MME) is terminated at the MeNB, so from the prospective of a core network, the MeNB can be regarded as a mobile anchor; and the SeNB is responsible for providing the user equipment with additional radio resources in addition to the MeNB.
In the dual connectivity scenario, frame structures applied to cells and/or carriers scheduled by different base stations may or may not be the same. For example, a Frequency Division Duplex (FDD) frame structure is used on the respective carriers scheduled by the different base stations for the user equipment; or a Time Division Duplex (TDD) frame structure is used on the respective carriers scheduled by the different base stations for the user equipment (wherein TDD uplink/downlink configurations applied to the respective carriers scheduled by the different base stations may or may not be the same); or an FDD frame structure is used on respective carriers in one frequency band for the user equipment, and a TDD frame structure is used on respective carriers in another frequency band for the user equipment.
In the dual connectivity scenario, the plurality of base stations which the user equipment is connected schedule their respective sets of downlink carriers separately, where data can be scheduled and transmitted for the user equipment in the following three options:
Option 1: bearers of the MeNB are routed directly to the MeNB from a gateway (e.g., a Serving Gateway (S-GW)); and bearers of the SeNB are routed directly to the SeNB from the gateway, that is, the bearers of the SeNB are not routed through the MeNB, as illustrated in FIG. 1;
Option 2: bearers of the MeNB are routed directly to the MeNB from a gateway; and firstly bearers of the SeNB are routed to the MeNB from the gateway, and then all the bearers are split by the MeNB to the SeNB, that is, the bearers of the SeNB are not split, as illustrated in FIG. 2; and
Option 3: bearers of the MeNB are routed directly to the MeNB from a gateway; and firstly bearers of the SeNB are routed to the MeNB from the gateway, and then a part of the bearers are split by the MeNB to the SeNB, whereas the other part of the bearers are still transmitted at the MeNB side, that is, the bearers of the SeNB are split, as illustrated in FIG. 3.
In the dual connectivity scenario, the user equipment may receive data on the sets of downlink carriers scheduled by the different base stations, so the user equipment needs to feed back Uplink Control Information (UCI) corresponding to the downlink carriers scheduled by the different base stations. However in the dual connectivity scenario, a solution to making a feedback by the user equipment for the downlink carriers scheduled by the different base stations has been absent so far.